KVM itself doesn't really do anything with clipboards. What's more likely happening here is that the RDP client used by this project (FreeRDP) supports the RDP

KVM itself doesn't really do anything with clipboards. What's more likely happening here is that the RDP client used by this project (FreeRDP) supports the RDP clipboard redirection feature. See: rdpclip.exe running in the target machine whenever you RDP.